Amorphous polyester resins that do not have a conspicuous melting point are used in the field of building materials such as decoration films for covering plywood, the field of food packaging materials such as food trays and blister packs, as well as for printed cards, magnetic cards and so on. Furthermore, polyester resins excellent in biodegradability such as polylactic acid are used in the field of agricultural civil engineering for materials such as flat yarns, nets, gardening materials, and pots for raising seeding, windowed envelopes, shopping bags, compost bags, writing materials, and miscellaneous goods. However, all of these applications are in the field of rigid mold products. These materials, being likely to be whitened when folded and deficient in flexibility, have not been used in soft applications or semi-rigid applications. There have been proposed various methods of adding a plasticizer as a technique to apply in the soft or semi-rigid field.
JP-B 2601737 proposes a technique in which by adding talc having a particular particle diameter and a hydroxy aliphatic glyceride by the respective particular ratios to an amorphous linear polyester resin, in the case of trays, blister packs and so on being stacked and stored, the detachability when these are peeled one by one (renesting properties) can be improved. Furthermore, JP-B 4-4652 proposes a technique in which by adding a particular ratio of a bis-amide compound to a biaxially stretched polyester film, the friction between films can be reduced and the traveling performance can be improved. Since these techniques intend to make a polyester resin composition that does not contain a plasticizer have a smaller friction coefficient between films and be easier in peeling, or easier in preventing resins from sticking onto rolls.
General purpose resins such as polyethylene, polypropylene, polyvinyl chloride, and polystyrene obtained from petroleum are used, because they are light weight, have excellent workability, and have physical properties such as durability, in various fields such as convenience goods, home electric appliances, automobile components, building materials or food packaging. However, when these resin products come to life's end and are discarded, the excellent durability disadvantageously works, that is, the decomposability in nature is inferior; accordingly, these may adversely affect the ecological system.
In order to overcome such problems, polymers that are thermoplastic resins and have biodegradability properties, including biodegradable polyester resins such as polylactic acid and copolymers of lactic acid and other aliphatic hydroxy carboxylic acids, and copolymers including aliphatic polyesters derived from aliphatic polyhydric alcohols and aliphatic polycarboxylic acids, and these units have been developed.
These biodegradable polymers, when placed in soil or seawater or within bodies of animals, owing to the operation of enzymes produced by microbes living in nature, start decomposing within several weeks, resulting in disappearance within from substantially one to several years. Furthermore, the decomposition products become lactic acid, carbon dioxide, water and so on, which are harmless to human bodies. Among the aliphatic polyesters, polylactic acid base resins are at present expected to be put into practical use and used in the field of agricultural civil engineeing materials such as flat yarns, nets, gardening materials, and pots for raising seeding, windowed envelopes, shopping bags, compost bags, writing materials, and miscellaneous goods. This is because L-lactic acid came to be mass-produced inexpensively from sugar obtained from sweet cone, yarn and so on by a fermentation method; the total carbon dioxide emissions are very small because raw material is a natural agricultural crop; and the toughness of such obtained polymers is large and the transparency is excellent. However, since the polylactic acid products are fragile, rigid and deficient in flexibility, applications thereof are limited to rigid molding articles. When it is molded in film, the flexibility is deficient and causes whitening when folded. Accordingly, it is not used at present in the soft applications or semi-rigid applications. As a technique applicable to the soft and semi-rigid field, methods of adding a plasticizer are variously proposed. For instance, a technique (JP-A 2000-302956) in which a plasticizer such as glycerin diacetyl monoaliphatic acid ester is added is disclosed. However, there are problems in that performances such as transparency and whitening resistance at folding are insufficient; furthermore, when a high temperature preservation test is applied assuming summer conditions, the transparency and the flexibility deteriorate remarkably, and the plasticizer bleeds out on a sheet surface.